The present invention relates to the cardiac pacemaker art, and, more particularly, to a pacer electrode system with an implanted electrode for use in controlling a demand pacemaker and/or for monitoring the cardiac electrogram.
Presently, demand pacemakers are controlled by sensing the intracardiac electrogram, particularly the R-wave portion thereof, and using it to determine whether the pacemaker stimulus is required to sustain normal cardiac rate. The intracardiac electrogram is sensed by means of the same electrode, or electrode pair in a bipolar system, that is used for pacing the heart, and because intracardiac electrodes are either introduced into the heart through small veins or are sutured to te exterior wall of the heart, practical restrictions are placed on the size of the electrode. A problem is presented by this situation in that the electrode size affects both the pacing and sensing functions. For pacing, it is desirable to minimize the electrode surface area to achieve high current density, but, conversely, the size of the electrode should be maximized for optimal sensing of cardiac potentials. As a result, in the past, electrode design has frequently been a matter of compromizing size to achieve a balance for both adequate pacing and sensing. An example of one solution for this problem is disclosed in U.S. Pat. No. 3,977,411.
However, in accordance with the present invention, it has now been determined that it is possible to overcome the size compromise problem and achieve improved sensing of cardiac potentials through the use of a separate electrode element, implanted near or in direct contact with the cardiac tissue, and positioned for sensing both R- and P-waves while avoiding interference from the pacing electrode. This separate element may be appropriately positioned with respect to the heart and the pacemaker casing, and have a comparatively large surface area, to optimize the electrical vector sensing of both R- and P-waves and to provide more stable sensing signals. The sensed signals may be fed to the pacer circuitry and used for improved atrial, ventricular or dual chamber demand pacemaker control or telemetered thereby to provide for the first time, noninvasive cardiac electrogram monitoring.
With telemetering from within the patient to an external analyzer for analysis, no skin or patient contact is required as in external cardiac electrogram monitoring. Consequently, currently-used EKG and rate dependent pacemaker follow-up techniques which require attachment of bracelets to the patient's extremities or pressure against the patient's wetted skin with three or four metal-footed conductive pickups are obviated along with their attendant disadvantages of time-consuming manipulation and artifact introduction.